1. Field of the Invention
The present invention relates to an electronic control apparatus, and to a memory apparatus of the electronic control apparatus, with the electronic control apparatus having a floating-point arithmetic function and performing various types of calculation and control operations by execution of a predetermined program.
2. Description of Related Art
In recent years, types of electronic control apparatus for performing functions such as controlling the engine or other equipment of a vehicle have come into use, whereby data expressed in floating-point representation are used in performing various types of processing, instead of data expressed in fixed-point representation. This is described for example in Japanese patent No. 2001-282505. Use of floating-point data enables a higher level of calculation accuracy to be attained than is possible with fixed-point data.
FIG. 8 shows the single precision storage format for data in accordance with the IEEE754 standard, which is one of the standards for floating-point data. As shown in FIG. 8, the single precision storage format consists of 4 bytes, i.e., a sign portion which is a single bit, an 8-bit exponent, and a 23-bit mantissa. Due to the fact that the mantissa is formed of 23 bits with this single precision format, it is possible to perform calculations to an accuracy of ½23 (≅0.0000001), i.e., with 7 digits below the radix point.
Usually with such a type of electronic control apparatus which must be executed using various types of control, the apparatus incorporates a ROM (read-only memory) for storing respective sets of map data of one or more maps. Each set of map data expresses relationships between a set of map points (i.e., input values for the map) and a corresponding set of map values (i.e., from which output results can be obtained, in response to map points being specified). Specifically, an output result is obtained in response to a specific map point, based on a map value which directly corresponds to that map point, or based on interpolation between a plurality of map values that are adjacent to the map point.
However when such a map is configured with all of the map data being stored as floating-point representation data, then the amount of stored data may become large. For example, if all of the map data are stored as single precision storage format data, then it is necessary to use 4 bytes of data for each map point and 4 bytes for each map value. Hence, the required ROM storage capacity becomes excessive. In recent years, due to the increasing complexity of control of vehicle engines, the amount of data which must be stored in such a ROM has increased accordingly, so that this problem is becoming more severe.